This invention pertains to a method and apparatus for polishing a workpiece.
A typical process for manufacturing a magnetic disk comprises the following:    1. An Al alloy substrate having an ID and an OD is provided.    2. The edges at the ID and OD of the substrate are chamfered.    3. The substrate is electroless plated with a NiP.    4. The NiP is polished. (See, for example, U.S. Pat. No. 6,149,696, issued to Jia, incorporated herein by reference.)    5. The NiP is textured. (See, for example, U.S. patent application Ser. No. 10/299,028, filed by Homola, incorporated herein by reference.)    6. One or more underlayers (e.g. Cr or a Cr alloy) are sputtered onto the textured NiP.    7. One or more magnetic layers (e.g. Co magnetic alloys) are sputtered onto the underlayers.    8. One or more protective layers (e.g. C or hydrogenated C) are deposited onto the magnetic layers. (See, for example, U.S. Pat. No. 6,565,719, issued to Lairson et al., incorporated herein by reference.)(Other layers, e.g. Ru intermediate layers, seed layers, and other types of layers are sometimes deposited at different points during the manufacturing process. See, for example, U.S. patent application Ser. No. 10/075,123, filed by Bertero et al., incorporated herein by reference.)
As described by Jia, substrates are often polished in planetary polishing apparatus. FIGS. 1A and 1B schematically illustrate in plan view and cross section, respectively, polishing apparatus 1 constructed in accordance with the prior art. Such apparatus comprises a substrate holder 2 having openings 3 for accommodating substrates 4 that are to be polished. (Substrates 4 have a centrally defined opening 4c therein.) Holder 2 is roughly circular, and comprises teeth 5 for engaging with a central “sun gear” 6 and for engaging with an outer gear 7. Gears 6 and 7 cause holder 2 to revolve around sun gear 6 (see arrow A) while simultaneously rotating about the center 8 of holder 2 (see arrow B). Upper and lower pads 9, 10 (mounted on rigid platens 11, 12 respectively) press against the upper and lower surfaces 4a, 4b of substrates 4. Slurry is provided between pads 9, 10 and substrates 4 to polish substrates 4.
At the conclusion of polishing, it would be desirable for the substrate 4 to have a profile as shown in cross section in FIG. 2. Unfortunately, substrates often emerge from the polishing process with a defect called “roll-off”, schematically shown by dotted lines R in FIG. 3, which extends into the data zone 4z. (Data zone 4z is where data will ultimately be recorded on the disk when it is finished.) One cause of roll-off is that substrates 4 have a thickness T4 that is greater than thickness T2 of carrier 2. Therefore, pads 9, 10 tend to push harder against the edges E of substrates 4, thereby causing roll-off. It would be desirable to prevent roll-off.